I. Field of the Invention
The present invention describes a rain-responsive wiper control system which responds rapidly to changes in moisture conditions, yet does not change modes of operation so frequently as to be subjectively erratic. As a result, the resulting system is simultaneously smooth and responsive--two characteristics not available in prior art wiper control systems.
I. Discussion of the Prior Art
A moisture sensing windshield wiper control system must necessarily employ some moisture sensing means. Referring to FIG. 1 of the drawings, this sensing means may, for purposes of illustration, be an optical sensor 1, such as is disclosed in the McCumber et al. U.S. Pat. No. 4,620,141 and the Teder U.S. Pat. No. 5,059,877 disposed on the inside surface of a windshield 2, within the path 3 swept by wiper blades 4. This moisture sensing means 1 may also be capable of sensing disturbances, such as shadows, as described in U.S. Pat. No. 5,059,877. For further purposes of illustration, the components of a moisture-sensitive wiper control system may be partitioned into the functional elements illustrated in FIG. 2. In FIG. 2, the output 10 of a rain sensing means 1 is coupled to the input of an analog-to-digital converter 11. A smoothing algorithm 12, implemented either in hardware or software, then actuates the wipers 4 in what it determines to be an optimum manner, by applying appropriate signals to a wiper motor 13, by way of a vehicle interface 14. Input from the driver concerning operating mode and desired system sensitivity is imparted to the vehicle interface 14 by means of a driver accessible switch 15, conventionally mounted on the steering column. A suitable vehicle interface has been disclosed in U.S. Pat. No. 5,239,244, which is also assigned to applicant's assignee.
In its most primitive form, the smoothing algorithm 12 may simply run the wiper motor 13 when the presence of moisture is detected, and many prior art rain-responsive wiper control systems posit this as a means of control (e.g. Noack, U.S. Pat. No. 4,355,271). This simple method suffers at least two drawbacks: 1) The area of the windshield sampled by the sensor is small compared to the windshield as a whole, thus tending toward erratic behavior arising from the random nature of the signal; and, 2) Even were the sample area of the windshield large enough, it is subjectively annoying to the driver of the vehicle for the wiper to actuate in an erratic manner, even if the rainfall itself is fluctuating in an erratic manner.
Thus some degree of moisture-sensor response smoothing is desirable. One method, realized with analog electronic components, is disclosed in the above-referenced U.S. Pat. No. 4,620,141. In U.S. Pat. No. 5,059,877, the smoothing function is refined and implemented in the software of a microcontroller. Acceptable system behavior is obtained from this method, but the resulting performance constitutes a tradeoff between smoothness and responsiveness.
FIG. 3 diagrammatically illustrates the prior smoothing method embodied in the '877 patent. Sensing means 1 produces a signal on line 10. It is coupled to a block 21 which takes the absolute value of the deviation of the signal 20 from its quiescent level. The resulting rain deviation signal 22 is coupled to a curve shaping means 23 which produces pulses 24 which are proportional to the degree to which the deviation signal 22 exceeds a threshold. This essentially amplitude-dependent pulse-signal on line 25 is coupled to an averaging means 26. The averaging means 26 produces an ongoing estimate 27 of moisture flow or flux. The circuit components 28-31 comprising the averaging means 26 respond to increases in signal amplitude more rapidly than to decreases. That is, the means 26 features asymmetric attack and decay rates. This and other prior methods of smoothing the response of the rain sensor have fallen short of optimum because of underlying principles which will be identified and discussed as the present invention is described.
A first limitation of prior art rain responsive systems is that methods used to determine the flow-rate (flux) of the rainfall are inadequate. The prior art approaches typically center around the duration or amplitude of signal excursions of the moisture-sensitive signal. The resulting signal is only loosely correlated to the actual flow rate of the rainfall. Koybayahi teaches (U.S. Pat. No. 4,542,325) that sensor amplitude may be integrated before comparison to a threshold, but this does not circumvent the effects of small sample size. Mangler teaches (German Patent DE 40 18 903 C 2) that the intensity may be derived from the period between successive detections of the sensing means, but this too may be expected to vary wildly. The underlying difficulty is that, while they are simple to implement, the primarily amplitude-dependent or period-dependent sensing methods of prior art automatic rain responsive wiper control systems do not estimate rain flow-rate (flux) as accurately as does a frequency-oriented method which, as will be described, is implemented in the preferred embodiment of the present invention.
Another recognized limitation of prior art rain responsive wiper systems is that, in general, quantities related to the rain flow rate are linear in nature, while most aspects of human perception tend to be logarithmic in nature. That is, amplitude excursions, estimated rate of rainfall, and time-based measurements are all linear quantities. While these linear representations are simple to implement in an electronic system, they are at odds with the nature of most aspects of human perception. For example, sound intensity and frequency are both perceived logarithmically by humans, as is light intensity. Human perception of time may also be considered to be logarithmic in nature. For example, the difference between wipe rates of one per second and two per second is considerable, while the difference between ten per second and eleven per second is not readily perceptible. In failing to address the nature of human perception, prior art rain responsive wiper systems inherently represent large changes in the perceived wipe period with merely small numerical changes in the contents of an internal register within its microcontroller potentially leading to resolution difficulties. It is reasonable to suppose that a microprocessor based system with a word size of, say, 8 bits is capable of providing a resolution more in line with human perception and, hence, can be made to behave in a manner more subjectively pleasing to humans.
A further limitation of known prior art systems is that they operatively contain a single time constant based on prior history of rain accumulation. The rate of wiper actuations should most properly be based upon the prevailing, long term (on the order of tens of seconds) conditions in which the vehicle is operating, as well as the shorter (on the order of seconds) term fluctuations of the sensed rain signal.
The system described in the Koybayahi patent effectively implements a short time constant, the effects of which are "forgotten" by the system after each wipe period. The aforereferenced U.S. Pat. No. 4,620,141 teaches that a time constant may be applied to the sensor output, and this will inherently permit the period of prior wipe accumulation history to be considered in determining an appropriate wipe rate. Similarly, Mangler teaches (German Patent DE 40 18 903 C 2) that the consideration of prior history can be a function of prior wipe-periods. This also has the effect of implementing a single time-constant response. None of the prior art systems maintains a separate signal or register value which corresponds to the prevailing long-term conditions experienced by the vehicle equipped with a rain sensor.